The present invention relates to treatment of solutions utilized in the metal finishing industries and, in particular, to a process wherein sludge produced during regeneration of the metal finishing solutions and containing a high percentage of calcium aluminate, among other solution constituents, is treated to remove undesirable characteristics thereof such that the sludge may be disposed of in a non-harazdous landfill.
Various metals, such as aluminum and the alloys thereof, are often pickled, milled, etched, cleaned and the like by various chemical solutions, such as sodium hydroxide (caustic soda) in an aqueous solution. These processes are generally referred to as metal finishing herein.
Typically in these finishing processes an exposed portion of metal, for example aluminum, is removed by reaction with a chemical milling agent such as sodium hydroxide. Lime may be added to the solution after the milling process to precipitate previously soluble aluminum salts as generally insoluble calcium aluminate thereby regenerating the milling solution by reforming sodium hydroxide. This regeneration process is thoroughly discussed in applicant's U.S. Pat. No. 3,607,482, PROCESS OF REGENERATION OF METAL TREATING SOLUTIONS, the disclosure contained in such patent being included herein by reference. The precipitate formed in the above discussed regeneration process actually contains a number of constituents which in general are referred to herein as a calcium aluminate sludge. Normally, the sludge contains approximately the following quantities as a percent by weight:
______________________________________ Calcium Aluminate 60-80 percent Metal Sulfides (Si, Fe, Cu, Mn, Mg, Zn) 0-10 percent Calcium Carbonate 4-8 percent Calcium Hydroxide 15-20 percent ______________________________________
It is noted that the above percentages are given as an example only and that these percentages can vary greatly. In addition, it is possible to remove the metal sulfides prior to the above discussed lime addition. The precipitated sludge is preferably removed from the chemical milling solution after the solution of lime by operation of a clarifier. The clarifier mother liquor consists heavily of sodium hydroxide in an aqueous solution along with other minor constituents including aluminum and sodium gluconate or the like. It is normal for the sludge to have some contamination from the mother liquor.
Because of continually upwardly spiraling costs related to safe operation of chemical disposal sites and increasing governmental regulations directed to disposal of chemical wastes, it has become increasingly difficult to afford and/or even find a suitable disposal site for the precipitated sludge. A major environmental problem associated with the sludge is that some of the constituents thereof may be leachable so as to eventually become dislocated from the disposal site and find their way into underground or surface water systems.
Not only are the leachable components of the sludge a problem, the pH of the sludge even without the leachable items is often above an acceptable level for disposal in a non-hazardous landfill. In particular, the calcium hydroxide produces a pH in the nature of 12.